1. Field of the Invention
The present invention relates to a micro-electromechanical system (MEMS) device having flexures with non-linear restoring force.
2. Description of the Related Art
In general, a MEMS device is obtained by processing a substrate and has a micro actuating element for moving on the substrate and a driving element for moving the micro moveable element on the substrate. The driving element has two electrodes opposite to each other to generate attractive force mainly by static electricity. In general, there are three types of actuating elements: one moves in a direction parallel to the substrate, another moves in a direction perpendicular to the substrate, and the other rotates with respect to the substrate within a predetermined range of angles.
For these movements, a moving electrode is prepared at a portion of the moveable element and a fixed electrode is prepared in a fixed position opposite to the driving electrode. In general, since in a MEMS structure, the distance between both electrodes is kept in the range of a few microns, very precise processing is required. An important defect sometimes occurring in MEMS devices is stiction of the moveable element to an adjacent fixed element.
FIG. 1 is a perspective view of a conventional micro switching device having a MEMS structure, as an example of a MEMS device which is very sensitive to the above defect. As shown in FIG. 1, an actuating stage 2 is placed over a substrate 1. The actuating stage 2 is supported by flexures 3 extending from four corners of the stage 2, and anchors 4 supporting the flexures 3.
The actuating stage 2 includes moving electrodes 2a and 2b at opposite sides and a contact point 2c between the moving electrodes 2a and 2b. Fixed electrodes 5a and 5b are placed underneath the moving electrodes 2a and 2b. Signal lines 6a and 6b for switching are positioned underneath the contact point 2c. Here, inward ends of the signal lines 6a and 6b are spaced apart from each other underneath the contact point 2c. 
In the switching device, the actuating stage 2 moves in a Z direction perpendicular to the substrate 1 by static electricity between the fixed electrodes 5a and 5b and the moving electrodes 2a and 2b. Here, when the actuating stage 2 moves toward the substrate 1, the contact point 2c contacts both signal lines 6a and 6b to allow an electrical connection between the signal lines 6a and 6b. 
FIG. 2 is a plan view of the actuating stage 2, the flexures 3 extending from the four corners of the actuating stage 2, and the anchors 4 supporting the flexures 3 in the conventional switching device. The actuating stage 2 and the flexures 3 are formed of metal as one body.
The conventional switching device has a disadvantage in that the actuating stage 2 easily sticks to the surface of the substrate 1 when the actuating stage 2 moves by electrostatic force between electrodes. This sticking mainly occurs if there is moisture or foreign matter between the actuating stage 2 and the substrate 1. Sticking of the actuating stage 2 may occur in use or during a manufacturing process.
Conventionally, a protrusion is prepared or a non-stick thin film is formed at the bottom surface of the actuating stage 2 in order to prevent such sticking. However, in the case of a micro switching device, switching occurs at the bottom surface of the actuating stage 2. Thus, a protrusion may increase contact resistance. Moreover, in the micro switching device having parallel electrodes, non-linear electrostatic force occurs between both electrodes. If the initial distance between the electrodes is reduced to a third, then the electrostatic force considerably increases compared to the restoring force of the flexures. As a result, when electrodes get to close, they stick together.
To solve the above-described problems, it is an object of the present invention to provide a MEMS device having flexure elements with non-linear restoring force which is capable of effectively preventing a moveable element from sticking and stably restoring the position of the moveable element.
Accordingly, to achieve the above object, there is provided a MEMS device having flexure elements with non-linear restoring force. The MEMS device includes a substrate, support elements formed on the substrate, a moveable element positioned over the substrate by the support elements to move relative substrate, flexure elements for elastically suspending the moveable element on the support elements, a driving element for causing the moveable element to move relative to the substrate, and repulsive elements for causing a sudden steep increase in the repulsive force of the flexure elements when the flexure elements supporting the moveable element are resiliently deformed by a predetermined amount during movement of the moveable element.
According to the present invention, the repulsive elements having a predetermined size are stoppers positioned between the flexure elements and static elements fixed on the substrate opposite to the flexure elements.
The stoppers may be positioned at portions of the static elements opposite to the flexure elements so that middle portions of the flexure elements contact the stoppers when the flexure elements are resiliently deformed by a predetermined amount. Also, the stoppers may be formed on middle portions of the flexure elements opposite to the static elements so that the stoppers contact the static elements when the flexure elements are resiliently deformed by a predetermined amount.
Preferably, the moveable element moves in a direction perpendicular to the plane of the substrate. The static elements may be portions of the surface of the substrate opposite to the flexure elements.